The two primary functions of a rear suspension system in a bicycle are to maintain tire contact with the ground during bumps and depressions and to isolate shock and vibration from the rider.
Many different rear suspension systems are presently on the market. Known systems include the high pivot design, low/forward pivot design, bottom bracket pivot design, unified rear triangle design and numerous multi-link designs.
A problem that all rear suspension designers have to address is the elimination of undesirable effects associated with rear suspension including those which have been loosely referred to by cyclists as "bio-pacing".
In recent times there has been an attempt to quantify "bio-pacing" in terms of neutrality (N-factor) and pedal feedback (P-factor). Both factors are functions of suspension geometry and also vary with gear selection. When a cyclist refers to "bio-pacing" they are usually referring to pedal feedback or P-factor.
N-factor is the tendency of the suspension to be extended or compressed by pedaling forces which are pulse-like in nature. Regardless of whether the suspension is compressed or extended by pedaling forces, this is wasted energy which causes the sprung mass to bob up or down with each pedal stroke.
A positive N-factor causes the suspension to extend under pedaling forces whilst a negative N-factor causes the suspension to compress under pedaling forces.
Whilst neither a positive N-factor or negative N-factor is desirable, a positive N-factor is believed to have some advantage during climbing due to the fact that the extension of the suspension tends to urge the tire into engagement with the ground as power is applied thereby improving traction during power application.
A positive N-factor also tends to make the suspension less active ie. if a bump is struck by the rear wheel during the application of power, the positive N-factor will tend to resist the compression of the suspension which would otherwise occur to absorb the bump. A very positive N-factor may result in the suspension "locking out" or "topping out" in which case the suspension may be totally inactive and will also be unable to extend further to follow depressions.
A negative N-factor, as explained above, means that the suspension is compressed by pedaling forces. This tendency is accentuated during out-of-the-saddle pedaling because the rider's tends to "bounce" on the bike and the downward weight is applied simultaneously with the pedaling force, both of which compress the suspension. When the suspension is compressed it tends to be stiffer and further suspension travel is limited thereby inhibiting the suspension's compliance to bumps.
P-factor is the tendency for the length of the top run of chain extending tangentially between the gear and chainring to vary during suspension action. This variation manifests itself in a forward or backward rotation of the pedals whilst the pedals are stationary, or in a speeding up or slowing down of the pedal rotation during actual pedaling.
If, due to geometry, the rear axle moves away from the bottom bracket during compression of the suspension, then the length of chain extending tangentially between the gear and chainring must increase and this increase will be compensated for by a rearward rotation or slowing of the pedals (positive P-factor). Of course, the pedals will then rotate forwardly or speed up when the suspension returns to its uncompressed state.
Conversely, if the rear axle approaches the bottom bracket during compression of the suspension, then the length of chain extending tangentially between the gear and chainring must decrease and this decrease will be compensated for by a forward rotation or speeding up of the pedals (negative P-factor). The pedals will then slow or rotate rear-hardly as the suspension returns to its uncompressed state.
A further factor which influences the variation of the length of chain extending tangentially between the gear and chainring (and hence pedal feedback or P-factor) is the relative sizes of the gear and chainring. In this regard, as the suspension compresses the chain tends to wind onto the gear and tends to wind off of the chainring. Due to the fact that the chainring is usually larger in diameter than the gear, a greater length of chain winds off the chainring than onto the gear and therefore the pedals must rotate forwardly to absorb the slack ie. there is a small negative P-factor effect caused by the winding/unwinding effect. This winding/unwinding effect differs in magnitude with different gear selections.
The unified rear triangle (URT) design referred to above has eliminated P-factor by combining the rear axle and bottom bracket in a single rigid unit. Thus, there is no variation of the dimension between the bottom bracket and rear axle and there is no winding/unwinding effect. For this reason the URT design is presently popular. The URT design, however, has new problems such as, for example, the fact that the distance between the bottom bracket and saddle is variable and this presents biomechanical difficulties.
The present invention provides an alternative rear suspension system which ameliorates P-factor. The present invention also provides a system in which N-factor may be simultaneously controlled.